The present invention relates to the field of semiconductor sensors, and in particular to a semiconductive sensor with a gas sensitive field effect transistor.
Gas sensors that utilize the change in work function of sensitive materials as the physical parameter have a fundamental development potential. The reasons for this relate to their advantages which are reflected in their low operating power, inexpensive fabrication and design technology, as well as the wide range of target gases. The latter can be detected with this platform technology since numerous different detection substances can be integrated in these designs.
FIG. 5 illustrates the sensor design based on the principle of a so-called suspended-gate field-effect transistor (SGFET). The bottom of the gate electrode, which is raised in this design, has the sensitive layer on which an electrical potential is generated in response to the presence of the gas to be detected based on absorption, which potential corresponds to a change in the work function of the sensitive material. As a rule, the signals have a level between 50 and 100 mV. This potential acts on the channel of the FET structure, thereby changing the current between the source and the drain (IDS). The coupling of this potential to the source-drain current is as a function of the gate capacitance C and of the ratios of channel width to channel length W/L:
                              I          DS                ∝                              W            L                    ·                      c            ⁢                                                  (                          c              ⁢                              :                            ⁢                                                          ⁢              area              ⁢                              -                            ⁢              specific              ⁢                                                          ⁢              gate              ⁢                                                          ⁢              capacitance              ⁢                                                          ⁢              in              ⁢                                                          ⁢              F              ⁢                              /                            ⁢                              m                2                                      )                                              [        1        ]            
The changed source-drain current is read out directly. Alternatively, the change in the source-drain current is reset by applying an additional voltage to the raised gate or to the transistor well. The additional voltage required represents the readout signal which is directly correlated with the work function change in the sensitive layer.
The SGFET has limitations in regard to the attainable signal quality due to the geometry required by the dimensioning of the FET structure. The available surface for coupling is limited by the process-technology-determined parameter W/L. Also related to the above factors, the signal quality is significantly affected by the introduction of the air gap between the gate and channel regions and the concomitant reduction of the gate capacitance. The height of the air gap must allow for sufficiently rapid diffusion of the gas and is in the range of a few p.m.
Use of the CCFET (capacitively controlled FET) design shown in FIG. 6 largely eliminates the limitations associated with the SGFET by providing a more flexible dimensionability. As a result, significant optimization is enabled in regard to signal quality. In the CCFET, the readout transistor 5, shown as source S and drain D, is controlled by a noncontacted gate (floating gate). Together with the opposing gate electrode which has the gas-sensitive layer, the noncontacted gate forms a capacitor arrangement. The surface of the capacitor arrangement is independent of the readout transistor and can thus be enlarged, thereby producing improved signal coupling.
However, the fact that parasitic capacitances are present between the floating gate 2 and the substrate or capacitance well 3 does have a disadvantageous effect even with the CCFET.
If the direct capacitances present in a gas-sensitive field-effect transistor are shown graphically in equivalent circuits, the diagrams of FIGS. 7 and 8 can be drawn up for the SGFET and CCFET variants. In FIG. 7, the SGFET clearly has a structure which is composed of a series connection of individual capacitances of air gap CL and those of the readout transistor CG.CSGFET=CL·CG/(CL+CG)
For a given sensitive layer and a given transistor, the air gap height is thus the single variable. In this case, no improvement of the signal coupling is possible by using an appropriate electrical control. A CCFET structure, for which a capacitive functional diagram is illustrated in FIG. 6, contains an additional electrode, the so-called capacitance well 3 located below the floating gate 2. As a result, the potential at the floating gate is determined by an expanded capacitive voltage divider which is formed from the air gap capacitance CL, the capacitance of the gate CG, and the capacitance occurring between the floating gate and that due to the capacitance well, as illustrated in FIG. 8. By enlarging the area forming capacitance CL, it is possible to reduce the effect of the parasitic gate capacitance while maintaining the air gap height. The gate capacitance of the readout transistor is—especially given appropriate dimensioning—negligible with good approximation relative to other capacitances. The capacitance well is used to shield the floating gate electrode and is accordingly connected to ground. Assuming the above preconditions, the potential at the floating gate UUF is:ΔUFG=ΔΦS·CL·(CL+CW+CG)/(CG+CW)  [2]The changes in the UFG are converted through the transistor characteristic directly into changes in the source-drain current IDS and in response to a given ΔΦs are a direct measure of the signal obtained with the gas sensor. Based on the introduction of an air gap with a height of a few μm, the result is:CL<<CG+CW  [3]
The result up to this point is a significant loss of signal.
The two variants according to the equivalent circuits of FIGS. 7 and 8 function by coupling the gas signal to the source-drain current IDS, used as an example of the measured quantity. The signal is degraded, however, due to the capacitances present. With the SGFET, this factor can be counteracted by increasing the W/L, and additionally in the CCFET by increasing the surface of the readout capacitance, such that in the limiting case a very large area is obtained whereby CG<<CW.ΔUFG∝ΔΦS·CL/CW  [4]
Using parameters possible in a standard CMOS process, in the CCFET a weakening of the signal on the order of 1:10 to 1:100 caused by the capacitive voltage divider must be expected with the operating method described above.
There is a need for an improved gas-sensitive field-effect transistor which largely eliminates interference effects.